This invention relates to a method for the electrolysis of an aqueous solution of an alkali metal chloride and an anode therefor. More particularly, the present invention is concerned with a method for the electrolysis of an alkali metal chloride which comprises conducting electrolysis of an aqueous alkali metal chloride in an electrolytic cell partitioned by means of a cation exchange membrane into an anode chamber and a cathde chamber, using a perforated plate anode in the anode chamber, and also is concerned with a perforated plate anode therefor which not only provides a low electrolytic voltage but also has a high durability.
An electrolytic process of an aqueous solution of an alkali metal chloride in which process a cation exchange membrane is used is gaining attention in the art, because this ion exchange membrane process is useful not only for overcoming the various drawbacks accompanying the two conventional processes for the electrolysis of an aqueous solution of an alkali metal chloride, namely, the mercury process and the diaphragm process, but also for saving energy. The noticeable features of the ion exchange membrane process are that neither mercury nor asbestos is used and therefore there is no fear of environmetal pollution, the cation exchange membrane used is capable of preventing the aqueous solution of an alkali metal chloride from diffusion from the anode chamber to the cathode chamber and therefore the purity of the alkali metal hydroxide produced is high, and the electrolytic cell is completely partitioned by means of a cation exchange membrane into an anode chamber and a cathode chamber and therefore the purity of each of the chlorine gas and the hydrogen gas produced is high. Further, the total energy cost as calculated from electric power and vapor, for example that in the electrolysis of an aqueous solution of sodium chloride, is lower than that in each of the mercury process and the diaphragm process. However, the rate of the cost of electric power in the proportionally variable cost in the total production cost is still high and is as high as about 40% in Japan. Taking into consideration the increasing price of petroleum oil in the future, the demand for the development of a new technique useful for lowering the consumption of electric power is increasing more and more in the art.
The anode currently used for the electrolytic method of an aqueous solution of an alkali metal chloride is mainly a metallic anode comprising a metal substrate of titanium or the like and a coating coated on the surface of said metal substrate, said coating being composed mainly of a precious metal oxide such as ruthenium oxide or the like. In the technical field of an anode, it is known that in electrolysis of an aqueous solution of an alkali metal chloride there is used an anode of a gas-removing structure in order to avoid elevation of the electrolytic voltage due to current shielding caused by the chlorine gas generated on the anode. In this known technique, such an anode of a gas-removing structure is devised so that the chlorine gas generated on the anode can readily escape from the anode chamber behind the anode with respect to the position of the cathode. Representative examples of such anode structure conventionally employed include an assembled structure in which a plurality of round metal rods each having a diameter of 2 to 6 mm are arranged in parallel at an interval of 1 to 3 mm and an expanded metal structure produced from a thin metal plate having a thickness of 1 to 2 mm.
In electrolyzing an aqueous solution of an alkali metal chloride by the mercury process using an anode of the gas-removing structure, the structural characteristics of the gas-removing structure have substantially no influence on the electrolytic voltage because there is present only an aqueous solution of an alkali metal chloride of low electrical resistance between the anode and the cathode as different from the present process using a cation exchange membrane having a relatively high electrical resistance. In the case of the diaphragm process, the asbestos diaphragm is pressed against the cathode. Further, the asbestos diaphragm is not selectively permeable to ions as different from the cation exchange membrane and, hence, there is not formed a desalted layer of high electrical resistance between the anode and the asbestos diaphragm. For the reasons stated above, also in the case of the diaphragm process, the structural characteristics of the anode have substantially no influence on the electrolytic voltage. Further, in the diaphragm process, there is generally employed a current density as low as 20 A/dm.sup.2. Furthermore, in the diaphragm process, there is generally employed, as an anode structure, the so-called expanded metal structure rather than the perforated structure produced by holing a thin plate having a thickness of 1 to 3 mm because the expanded structure can be produced at low cost due to the reduction in quantity of the titanium substrate material required. In industrial practice, there is usually employed an expanded metal anode which is produced by forming 10 to 30 mm--long cuts in a 1 to 2 mm--thick titanium plate, followed by 1.5 to 3 times expansion.
As different from the above-mentioned conventional two processes, in the ion exchange process, due to the selectivity to cations of the cation exchange membrane, the cation transport number in the cation exchange membrane is larger than that in the electrolytic solution in the anode chamber. For this reason, there is formed a desalted layer over the surface of the cation exchange membrane on the side of the anode. The desalted layer is extremely high in electrical resistance. Therefore, as proposed in Japanese Patent Application Laid-Open Specification No. 68477/1976, the electrolysis is conducted while maintaining the inner pressure of the cathode chamber at a level higher than that of the anode chamber so that the spacing between the anode and the cation exchange membrane can be reduced. The reduction of the spacing between the anode and the cation exchange membrane serves not only to lower the electrolytic voltage as the effect of said reduction itself, but also causes the desalted layer to be continuously, forcibly agitated by the action of the chlorine gas generated on the anode so that the thickness of the desalted layer can be considerably reduced, leading to further lowering of the electrolytic voltage. However, in the ion exchange process, there still remains unresolved such a problem that the current distribution in the cation exchange membrane often tends to be non-uniform so that the occasional elevation of electrolytic voltage and the deterioration of the cation exchange membrane for a short period of time cannot be avoided.
With a view to developing a new method overcoming the above-mentioned disadvantages, the inventors of the present invention have made extensive and intensive investigations. More specifically, the inventors have made such an investigation that the ion exchange process is carried out by adding a small amount of ions of radioactive isotope Ca.sup.45 to the electrolytic solution in the anode chamber to determine the distribution of Ca.sup.45 ions in the cation exchange membrane at the time when the Ca.sup.45 ions pass through the cation exchange membrane, together with the alkali metal ions. As a result, it has been found that not only the distribution of the Ca.sup.45 ions in the ion exchange membrane, that is, the current distribution in the ion exchange membrane but also the electrolytic voltage widely varies heavily depending on the structure of the anode. It has also been found that when the anode having, on its surface, convex and concave portions such as the conventional expanded metal anode is used, only the convex portions of the anode are contacted with the cation exchange membrane and therefore the current is caused to be concentrated only in the portions of the cation exchange membrane which correspond to the convex portions of the anode. Consequently, the current distribution in the cation exchange membrane becomes non-uniform, leading to not only elevation of the electrolytic voltage but also acceleration of deterioration of the cation exchange membrane. For obviating such drawbacks, it is advantageous to employ a flat type anode. However, with the simple flat type anode, it is impossible to remove the chlorine gas generated on the anode from the anode chamber behind the anode with respect to the position of the cathode, leading to elevation of the electrolytic voltage. Thus, it has been found that, in the ion exchange membrane process, the perforated plate anode is effective for obviating all the drawbacks as mentioned above. The present invention has been made based on such novel findings.
Accordingly, it is one and a primary object of the present invention to provide a method for the electrolysis of an aqueous solution of an alkali metal chloride in an electrolytic cell partitioned by means of a cation exchange membrane into an anode chamber and a cathode chamber, which enables the current distribution in the cation exchange membrane to be extremely uniform, thereby not only avoiding elevation of the electrolytic voltage but also prolonging the life of the cation exchange membrane.
It is another object of the present invention to provide a perforated plate anode for use in a method of the above character, which not only provides low electrolytic voltage but also has high durability.